Variation in the diversity and composition of benthic taxa as a function of distance offshore, depth and exposure in the Spermonde Archipelago, Indonesia

The aims of this study are to compare cross-shelf variation in diversity and community composition of four benthic taxa (sea urchins, sponges, corals and foraminifera) in the reefs of the Spermonde Archipelago, Indonesia, and relate this variation to cross-shelf environmental parameters, i.e. distance offshore, depth and exposure to oceanic currents. Rarefied species richness and Shannon's H′ varied unimodally with the distance offshore and depth for all taxa (except sea urchins) and were highest at intermediate distances offshore and depths. There was no significant association between evenness and distance offshore. Evenness did, however, vary with depth and exposure. Overall, models using distance offshore, depth and exposure explained most of the variation in sponge and coral diversity but substantially less variation in foraminifera diversity. Variation in composition depended on distance offshore, depth and exposure although the relative importance of these variables differed among taxa. Depth was the most important parameter for sponges, corals and foraminifera but not for sea urchins. Sponge and coral assemblages differed markedly from in-to-off-shore but there was relatively little variation in foraminifera assemblages with most foraminifera species having broad cross-shelf distributions. Foraminifera assemblages on the contrary differed markedly between exposed and sheltered reef environments; species in exposed reef environments, for example, consisted of species with distinct strategies for coping with the increased hydrodynamic energy. In summary, this study shows that benthic taxa in the Spermonde are distributed along continuous gradients (in-to-off-shore and shallow-to-deep) in addition to occupying discrete habitats related to reef exposure.